Permanent magnet electric motors and generators are well known and understood. Typically, such permanent magnet machines include a rotor formed, at least in part, from a magnetic material such as Samarium-Cobalt. Electric windings on a stator about the rotor are used to carry current that either generates a magnetic field or is the result of a magnetic field about the rotor. As a motor, current through the windings induces the rotating magnetic field, which in turn applies a torque to the magnetic portion of the rotor causing it to act as motor. Similarly, as a generator, torque applied to the rotor, results in a rotating magnetic field that induces a current in the windings.
Such electric machines provide significant benefits over synchronous machines, squirrel cage motors and other types of electric machines. Significantly, permanent magnet machines do not require brushes; are relatively light; use conventional and developed electronics to generate any required rotating magnetic field; and can act as:both motors and generators.
In view of these benefits, such machines appear well suited for aircraft applications. Particularly, such machines would appear to lend themselves for use as starters and generators within a turbine engine.
Conveniently, such machines can be connected directly to the engine shaft. When required, generated electricity can be rectified and filtered using conventional lightweight electronics. When DC currents are required, as in traditional aircraft applications, the speed of rotation and frequency of generator output does not need be controlled. Heavy gearing is therefore not required. Operating as motors, such machines can act as starters.
Disadvantageously, however, machines coupled to such engines can potentially generate extreme power limited only by the power of the turbine engine driving the rotor of the machine. Unabated, generation of such electric power can result in extreme heat, particularly in the stator windings, that may cause the motor to melt and potentially burn. This is clearly undesirable. Obviously, current provided by the machine to interconnected electrical equipment may be limited by fusing the interconnected equipment or even the electronics used to rectify or regulate AC currents. However, such fusing will not react to short circuits internal to the machine. While unlikely, such short circuits might, for example, occur in the stator windings. Should this happen, a permanent magnet machine will invariably overload and overheat causing damage to the machine, and perhaps even to the associated engine. In the extreme case, this may cause the main engine to fail as a result of the high temperature of the engine shaft coupled to the motor. Similar problems may be manifested in other types of electric machines.
Accordingly, an improved electric machine that is thermally protected is desirable.